Researchers studied 10 active young men. For three days, the men ate an energy-balanced diet--meaning calories in matched calories out. Then, still on the diet, each guy spent two days living in a sealed chamber, so researchers could monitor oxygen and CO2 levels—a way of exactly calculating calories burned.

Just hanging out watching TV, the guys burned 2,200 calories a day. But they burned 200 more on days when they did five 30-second sprints on an exercise bike. Now, these were not laid-back efforts. The guys pedaled as hard as they could, and had to take four-minute rest breaks in between.

Previous studies show that sprint training like this improves the body's response to blood sugar, possibly helping to stave off diabetes. And the researchers say it may be an alternative to endurance exercise for burning calories. That is, if you don't mind really feeling the burn.